The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.
Increased soil organic matter content confers productivity benefits to agricultural soil. Such benefits include improving soil structure by increasing aggregate stability, improving water holding capacity, improving soil nutrient content and availability to plants and conferring resistance to root disease. Methods to increase soil carbon can provide commercial benefits to crop and grain producers by producing higher yields for lower input costs and by sequestering carbon in soil.
Soil organic matter originates with carbon dioxide from the atmosphere becoming transformed by plant photosynthesis into plant biomass. Such plant biomass becomes an energy source for soil microbes which break down the organic matter by oxidation, leaving residues which are refractory to further decomposition and thus accumulate in soil. The progressive accumulation of refractory and stable organic matter increases the amount of soil carbon in a form which improves both the structure and the fertility of soils.
Studies of the biochemical mechanisms involved in the oxidative breakdown of plant residues have shown that woody residues containing substances such as suberin and lignin are not easily oxidised. Organisms capable of breaking down such refractory substances require high energy catalytic reactions to enable ring-cleavage, wherein the energy available in phenolic residues becomes available for metabolism. Such organisms typically belong to higher orders of soil microbes, including the higher fungi Dikarya, and higher bacteria Actinobacteria. These microbes have more sophisticated growth requirements than the lower orders of soil microbes such as the simple bacteria Proteobacteria and Firmicutes. Whilst the lower orders of microbes can grow on simple substrates such as sugar, higher orders of microbes require growth factors which include vitamins and higher molecular weight organic substrates. In a natural soil, the growth factors required by higher microbes are typically provided by the biomass and detritus produced by the lower microbial orders.
Soil priming is a phenomenon wherein when organic matter is added to soil it stimulates microbial respiration, often to the point where the total amount of organic matter oxidised exceeds the amount added. In the earliest days, the organic additions were things like leaf litter, straw or manure. In the last five or so years, researchers have added progressively smaller amount of purer substances like glucose and amino acids. It transpires that the initial burst of activity comes from the simple bacteria like Proteobacteria, which ecologically are k-strategists—meaning they grow rapidly feeding off simple media so can dominate the overall microbial population. This activity abates over time, and R-strategists take over. R-strategists are microbes with more sophisticated growth requirements, generally oligotrophs which can survive with lower energy availability and generally have much higher biodiversity than k-strategists.
Techniques such as metagenomics (the study of genetic material recovered directly from environmental samples) have been developed to study soil microbial ecology. Metagenomics typically uses shot-gun sequencing or pyrosequencing to determine the nature of DNA extracted from microbes in soil, allowing the production of a profile of microbial diversity in a natural sample. When soil microbes are grouped according to their phylogeny, metagenomics shows that many taxa are present within each of the major groups, and the proportionality of the groups and the taxa within those groups changes according to the physical, chemical and environmental attributes of the soil.
Metagenomics has shown that the cultivation of soils for agricultural production, and the application of fertilisers and other agricultural chemicals, causes profound alteration to the microbial populations in soil. Agricultural use generally causes a relative increase in the number of individuals and taxa in the lower orders of soil microbes and a decrease in the numbers and taxa of higher orders of soil microbes. Furthermore, the clearing of natural vegetation and the use of land for agriculture causes a substantial decline in soil organic matter. As it is soil organic matter which provides the growth substrate for soil microbes, the clearing of land for agriculture can cause a significant loss of microbial diversity.
It is against this background that the present invention has been developed.
The present invention seeks to provide the consumer with a useful or commercial choice of soil additive for the improvement of soil structure and/or fertility through the enhancement of net plant productivity